How to render a curve without creating meshes?
You could use OpenGL render (the button in the 3D View header) in Wireframe view mode. Every curve is rendered as a line without the need for any beveling. Animations can also be OpenGL rendered.

It is very fast. But there is no possibility to set the line width. It's always 1px.

To support transparency you have to set the shading mode to GLSL (press N in 3D View and look for Shading).